History and presenting complaint

Exercise-induced dehydration/exhaustion is most commonly recognized as a condition that requires medical intervention in horses exercising for protracted periods in hot environments. It is most commonly observed in horses competing in endurance rides and less frequently in horses undertaking three-day events or combined training competitions. The presenting complaint is severe depression, and lack of thirst and appetite despite apparent dehydration.

Physical examination

Clinical signs observed in horses suffering exercise-induced dehydration/exhaustion include depression, anorexia, lack of thirst despite persistent dehydration, persistently elevated rectal temperature, heart rate and respiratory rate.3,4 Other clinical signs related to dehydration and poor cardiovascular function include dryness of mucous membranes (gums, conjunctiva), delayed capillary refill time, decreased pulse pressure, and persistent skin-fold test. Dry feces, minimal urine production, poor gastrointestinal motility, poor anal tone, ileus, and colic may also be observed. Other less common signs associated with dehydration and exhaustion include cardiac arrhythmia, muscle cramps, and/or synchronous diaphragmatic flutter.

The severity of dehydration can be subjectively assessed based on the clinical signs observed (Table 40.1). Dehydration below 5% is not detectable clinically, whereas acute dehydration above 10–12% is considered incompatible with life.

In endurance competitions, horses are evaluated at regular intervals in an attempt to assure that only those capable of safely continuing to exercise remain in the competition. At each mandatory rest period (‘vet gates’), fitness to continue is assessed based on examination of general appearance, attitude, gait, pulse and respiratory rates, and evaluation of dehydration, capillary refill time, color of mucous membranes, and gastrointestinal motility. The cardiac recovery index is performed routinely in some countries during the veterinary examination at a mandatory stop. The cardiac recovery index involves taking a resting heart rate, the horse is then trotted 30 m (33 yards) out and back and the heart rate is taken again 1 minute after starting the trot out. Generally, if the resting heart rate is low (50 bpm or less), and the heart rate taken after trotting is equal or lower than the resting heart rate, the horse is considered fit to continue.5,6 Close examination of horses at the mandatory stop and use of the cardiac recovery index may help to identify those horses with significant dehydration/exhaustion, as it has been suggested in a recent study of a 160-km endurance ride in Western Australia.⁷

Laboratory examination

Routine hematology reveals alterations related to dehydration and stress, which include increased plasma total protein, packed cell volume, red blood cell count, and hemoglobin; and the total white blood cell count and differential generally show neutrophilia and lymphopenia.8–10

Plasma electrolyte concentrations in dehydrated/exhausted horses are variable. Moderate hypochloremia is most commonly observed after endurance exercise in exhausted and non-affected horses. However, the degree of dehydration and hypochloremia, with consequent metabolic alkalosis due to increased strong ion difference (SID), is generally more pronounced in affected horses. Although the characteristic acid-base abnormality is hypochloremic metabolic alkalosis, it is also reported that the fastest horses in an endurance race may develop mild metabolic acidosis due to lactate production,9,11 which probably results in a mixed acid-base disorder with concurrent lactic acidosis and hypochloremic metabolic acidosis. In a recent study of a 120-km endurance race, most horses had complex acid-base imbalances characterized by mild strong ion alkalosis (due to hypochloremia attenuated by hyperlactacidemia), non-volatile buffer acidosis and compensatory mild respiratory acidosis.¹² Other abnormalities can include hypokalemia, hypocalcemia, hypomagnesemia and hypo- or hypernatremia. Substantial decreases in total body electrolyte content due to sweating will be poorly reflected by changes in plasma electrolyte concentrations, because fluid and electrolyte losses are isotonic and of similar composition to plasma. Plasma electrolyte concentrations observed in horses performing endurance rides and three-day events are shown in Table 40.2 and Table 40.3.

Plasma glucose concentrations in dehydrated/exhausted horses are quite variable. Hyperglycemia can occur in exhausted horses and those with heat-stroke due to high concentrations of stress hormones, such as the catecholamines and cortisol. Hypoglycemia can also occur during protracted exercise because of depletion of liver glycogen, which will lead to fatigue and altered mentation due to lack of glucose availability to neurons.

Dehydration and possibly renal disease may be apparent by elevation in blood urea nitrogen and creatinine concentrations in horses performing protracted exercise. If urine can be obtained, it may be highly concentrated due to severe dehydration, or may be poorly concentrated despite severe dehydration in cases of ischemic renal tubular damage. The use of urinary specific gravity, urinary to plasma creatinine ratios and fractional excretion ratios of electrolytes in the diagnosis and management of acute renal failure is discussed below under the heading ‘Acute renal failure’.

Plasma concentrations of creatine kinase (CK) and aspartate aminotransferase (AST or GOT) may be elevated as a consequence of recent muscular activity, and in case of exertional rhabdomyolysis these enzymes are elevated 20–100-fold or more.

Intravenous fluid therapy

Intravenous fluids are classified as crystalloids or colloids. Crystalloid fluids are solutions containing electrolyte and non-electrolyte solutes capable of distributing among the body compartments. Colloid fluids contain large-molecular-weight particles, which do not normally pass through capillary membranes, and some smaller more diffusible particles. Colloid fluids include dextrans, glucose polymer mixtures of low (dextran 40) or high-molecular-weight (dextran 70 and dextran 75), gelatins, modified bovine collagens (oxypolygelatin), and hydroxyethyl starch preparations (hetastarch and pentastarch). Colloid fluids are indicated in those patients suffering increased capillary permeability and hypoproteinemia. Exhausted and dehydrated athletic horses will not generally require colloid fluids. Therefore, the following discusses the administration of crystalloid fluids that are used commonly in field situations for treatment of athletic horses.

The ionic composition, pH and osmolality of commonly used crystalloid fluids are presented in Table 40.5. The composition of polyionic fluids resembles that of extracellular fluid. Note that Ringer’s solution and 0.9% saline solution have a relative excess of sodium, and especially chloride, when compared to extracellular fluid concentrations. Exhausted/dehydrated horses generally have greater deficits of electrolytes relative to water (i.e. hypotonic dehydration) because fluid loss (sweat) is isotonic to slightly hypertonic and fluid intake (fresh water) is hypotonic. Therefore, the relative excess of sodium and chloride in Ringer’s and saline solutions works to our advantage in treating exhausted/dehydrated endurance horses.

Traditionally it was considered that anions used as buffers in some fluids are added as a source of base because their metabolism in the body yields bicarbonate, and hence their mild alkalizing effect:

Lactate :C3H5O3−+3 O2→2 CO2+2 H2O+HCO3−

Acetate :C2H3O2−+2 O2→CO2+H2O+HCO3−

however, a more accurate explanation is that fluids containing lactate and acetate are mildly alkalinizing because of their SID being close to or higher relative to that of plasma, as well as hemodilution and consequent decrease in non-volatile weak acids (A_tot, i.e. albumin, globulin and phosphates). In contrast, isotonic saline and Ringer’s solution are acidifying solutions because of their low SID relative to that of plasma (see Table 40.5).

Horses involved in endurance rides and those in the cross-country test (speed and endurance test, second day) of a three-day event competition will commonly suffer substantial fluid and electrolyte depletion, and hypochloremic metabolic alkalosis is the characteristic acid-base disorder. On occasion, in an attempt to dissipate heat, some horses have persistently elevated respiratory rate, which may lead to hypocapnia (low CO₂ tension in the blood) and mixed respiratory and metabolic alkalosis. Therefore, the primary goal is to correct the volume deficit with chloride-rich fluids, such as Ringer’s solution or 0.9% saline solution supplemented with potassium and calcium. A suggested amount of calcium supplementation is 10–20 mL of 23% calcium gluconate per liter of saline solution (10–20 mEq Ca²⁺/L or 5–10 mmols Ca²⁺/L). It is contraindicated to administer potassium at rates higher than 0.5 mEq/kg/h because of potential for cardiotoxicity. Unless cardiovascular function is closely monitored, potassium supplementation in intravenous fluids should not exceed 15–20 mEq/L. Ringer’s solution is preferred over lactated Ringer’s solution because, as mentioned above, the electrolyte composition and resulting SID is such that it may have a mild alkalinizing effect, which is contraindicated in an already alkalotic patient. However, in dehydrated horses the volume of fluid administered is more important than the actual acidifying/alkalinizing nature of the fluid, and one should not forego fluid administration if the only available fluids are alkalinizing, such as lactated Ringer’s solution.

Volume of intravenous fluids and rate of administration

In general, calculation of the volume of intravenous fluids to administer is based on estimation of daily maintenance requirements, current fluid deficits and assessment of ongoing losses. However, in practical terms, maintenance requirements and ongoing losses can be ignored in exhausted/dehydrated horses unless the horse develops diarrhea, ileus with abundant nasogastric reflux or polyuric renal failure. The goal of intravenous fluid therapy in exhausted/dehydrated horses is to replace water and electrolyte deficits to improve cardiovascular function and allow the horse to regain homeostasis, and to stimulate normal food and water intake.

Horses engaged in prolonged strenuous exercise events, like endurance competitions of 80 to 160 km or more, are reported to lose 4–8% of body weight (as much as 10% in a few horses) during the competition.10,15–17 Horses undertaking the speed and endurance test of a three-day event or a combined test may lose an average of 2–4% of body weight, which increases to up to 6–9% in a few horses in certain competitions.^18–21 However, sweating rates and consequently fluid and electrolyte losses depend upon environmental conditions, and horses exercising in hot or hot and humid conditions develop greater deficits when compared to more temperate conditions.²² If 90% of the body weight loss is fluid loss, then an estimated 20–40 liters of fluids are lost in a 450-kg horse (≈1000 lb) (Table 40.6). A simple estimation of the fluid deficit can be calculated as:

Table 40.6

Fluid deficits (L) depending on body weight (kg) and estimation of dehydration (%)

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Tags: Equine Sports Medicine and Surgery

Jun 18, 2016 | Posted by admin in EQUINE MEDICINE | Comments Off on Abnormalities of body fluids and electrolytes in athletic horses

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